Micromechanical Cohesive Force Measurements between

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Micromechanical Cohesive Force Measurements Between Precipitated Asphaltene Solids and Cyclopentane Hydrates Shane A Morrissy, Vincent W Lim, Eric F May, Michael L. Johns, Zachary M. Aman, and Brendan Francis Graham Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01427 • Publication Date (Web): 28 Aug 2015 Downloaded from http://pubs.acs.org on August 30, 2015

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Micromechanical Cohesive Force Measurements Between Precipitated Asphaltene Solids and Cyclopentane Hydrates Shane A. Morrissy, Vincent W. Lim, Eric F. May, Michael L. Johns, Zachary M. Aman, Brendan F. Graham* School of Mechanical and Chemical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia  Keywords: Asphaltene, hydrate, cohesion, adhesion, agglomeration, micromechanical force

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Abstract Asphaltenes are the heaviest and most polar class of compounds in crude oil, which may precipitate out of solution due to changes in the pressure, composition or temperature. During production, aggregation between asphaltene solids may lead to viscosification of the oil phase and/or deposition of the solids on the flowline wall. This study presents the first measurement of asphaltene interparticle cohesive forces using a micromechanical force (MMF) apparatus, which is similar to that used previously to investigate gas hydrate interparticle cohesion. Asphaltene solids were precipitated from two crude oils, and cohesive force measurements were performed for particle pairs with diameters ranging from 100 to 200 microns. In air, the measured cohesive forces between the asphaltene particles were approximately one half of those measured between hydrate particles in cyclopentane-saturated nitrogen vapor. Asphaltene cohesive force was measured in liquid cyclopentane, to provide a comparison against cyclopentane hydrate; in the liquid phase, the asphaltene cohesive forces were one order of magnitude smaller than the cohesive forces between cyclopentane hydrates. In addition, the hydrate-asphaltene adhesive force in liquid cyclopentane was measured to be of the same order of magnitude as that of hydrate particle cohesion; this result suggests the potential for asphaltene-hydrate solid aggregation as a potential flow assurance risk in oil and gas production flowlines.


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1 Introduction Asphaltenes are a core solubility class in petroleum fluids defined by their high density and polarity, insolubility in light aliphatic hydrocarbons, and solubility in aromatic solvents.1,

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Changes in system pressure, temperature or composition may lead to the precipitation of these high molecular-weight species from the oil during production.3,

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Of the hydrocarbon-based

solids associated with the production of oil and gas, the thermodynamics and precipitation kinetics of asphaltenes are the least well-characterized.2 Nevertheless, the aggregation/deposition behavior of asphaltenes are an important flow assurance concern, impacting both system design and operation. Asphaltene solid precipitation may result in a growing deposit within the reservoir rock, on the flowline wall during transport, during enhanced oil recovery, or in surface production facilities; asphaltene deposition in all scenarios introduces a safety hazard, decreases production rates, and limits operational efficiency.5, 6, 7, 8 If asphaltenes remain in solution, they may help stabilize water-in-oil emulsions,9 leading to increases in viscosity and/or problematic separations of the oil and water downstream.

In oil and gas production, asphaltene precipitation is generally caused by a decrease in pressure from the reservoir condition. Figure 1 shows a pressure-temperature curve for a general petroleum system, where the reservoir conditions exist at a pressure where asphaltenes remain dissolved in the liquid petroleum phase. As the system pressure and temperature decrease below the upper asphaltene precipitation envelope, the reduction in the oil’s solvation effectiveness means asphaltene components can begin to precipitate as a solid phase.6,

8, 10

If the system

pressure and temperature conditions continue to decrease the system can then pass through the wax and hydrate phase boundaries, leading to wax and hydrate formation.


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Figure 1. Example upper precipitation envelope for asphaltenes and phase boundary for waxes and hydrates, adapted from Amin et al.11 There will be significant variability in this asphaltene precipitation envelope for different crude oil sources.

The propensity for asphaltene to precipitate in production systems is not a simple function of asphaltene concentration in the liquid petroleum phase. For instance, oils with low asphaltene content have been reported to exhibit severe deposition problems during production, while other oils with high asphaltene concentrations have exhibited none.3,12,13 The production of Boscan oil in Venezuela has not been reported to result in deposition challenges, despite the oil having an asphaltene content of 17.2 wt%. Conversely, production of the Hass-Messaoud oil in Algeria has been reported to result in severe and periodic deposition problems, even though it has an asphaltene content of only 0.15 wt%.14 De Boer et al.15 proposed that light crudes with low asphaltene content, which are also undersaturated with gas, may result in high asphaltene supersaturation as the system pressure decreases.


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The description of asphaltene aggregation and deposition risk to date has largely relied on qualitative evidence and characterization; the reversibility and kinetic rate of asphaltene precipitation are not well understood. Hammami et al.6 have suggested that asphaltene precipitation caused by a decrease in fluid pressure is highly reversible; Joshi et al.8 support this argument and further suggest a re-dissolution timescale of minutes for small fractions. On the other hand, several authors have conversely proposed that asphaltene precipitation is irreversible in petroleum fluids if the operating pressure and temperature decrease too far below the lower precipitation envelope.16, 17, 18 Arguably, there are two main reasons for the limited quantitative understanding of asphaltene solubility in the liquid petroleum mixture: (i) the complexity of oil as a mixture and (ii) the difficulty of establishing what an asphaltene is at a molecular level.

The results of fluorescent time-dependent depolarization measurements have suggested asphaltenes possess a common characteristic of small conjugated aromatic rings; this observation has resulted in the proposal of two molecular architectures called island and archipelago.5 The island architecture consists of a central poly-aromatic nucleus with paraffinic, naphthenic and polar heteroatomic functionalities branching off the central nucleus. The archipelago architecture consists of crosslinked conjugated aromatic rings with the paraffinic, naphthenic and polar heteroatomic functionalities dispersed amongst the structure. These two molecular models have been proposed to satisfy the observations of high molecular weight and polyaromatic functionality.3, 5, 10, 19, 20 Over the last decade, limited experimental evidence has suggested that the island model of asphaltene chemical structure is an accurate first approximation,12, 19, 20, 21 although insufficient evidence has been presented to enable a definitive basis for asphaltene molecular structure.


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Due to the large variance in possible asphaltene chemical structures and functionality,2 asphaltenes are commonly characterized based on the bulk properties of the precipitated solid. Usually, the precipitation is done with a sample of the dead oil at atmospheric conditions that is diluted in an excess of a light alkane solvent. The choice of solvent can vary and in principle can produce an asphaltene precipitate with variable properties; it is necessary to indicate in laboratory studies which solvent was used to precipitate the asphaltene. As an example of this nomenclature, hexane or (C6)-asphaltenes represent the petroleum fraction that is insoluble in normal hexane and soluble in toluene.2, 3, 7, 22, 23, 24 Asphaltenes formed by this method may be more generally called “laboratory asphaltenes,” while solids formed in production systems via depressurization may be referred to as “field asphaltenes.”8 Laboratory asphaltenes have been shown to provide a repeatable basis for experimental investigation, but may not fully represent the chemical diversity of field asphaltenes.

From a flow assurance perspective, the aggregation and deposition properties of solid asphaltene particles are of commensurate importance to asphaltene solubility. Attempts at modeling asphaltene deposition and flocculation by authors such as Eskin et al.25 and Maqbool et al.26 invoke population balance models that are dependent on a collision efficiency term. The collision efficiency term allows for collisions between particles that do not result in aggregation, and it is estimated from experimental data26 involving aggregate growth. The work presented here provides a fundamental complement to the study of aggregate growth, by measuring the cohesive force between asphaltene particles. While much effort has been expended to develop a quantitative understanding of gas hydrate cohesive forces to enable prediction of agglomeration


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and the potential of forming a hydrate plug,27,

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limited studies are available to describe the

aggregation or deposition potential for precipitated asphaltene solids. Knowledge of the cohesive force between solid particles in a flowing oil phase is central to quantitative estimates of aggregate size, slurry viscosification and the probability of forming a blockage.29 For example, Sinquin et al.30 proposed a slurry viscosity model for hydrate aggregates suspended in crude oil where the cohesive force between hydrate particles was balanced against the shear stress exerted by the flowing continuous phase to estimate the maximum stable aggregate diameter:

(1)

where dA is the aggregate size, dp is the particle size, Fa is the interparticle cohesive force, Φ is the volume fraction of solids, Φmax is the maximum volume fraction of solids in the pipeline, f is the aggregate fractal dimension, µ0 is the viscosity of the dispersing liquid and γ is the shear rate. The model proposed by Sinquin et al. combines models proposed by Mills in 1985 (Equation (2)30 and Potanin in 1990 (Equation(4)).30 These individual physical interpretations of fluid behavior are connected via Equation (3).30 For a given level of shear stress imposed by the flowing fluid, the slurry viscosity is directly proportional to both the maximum stable aggregate diameter and interparticle cohesive force. (2)

(3)


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(4)

This in turn leads to the localized build-up of solids and potential risk of deposition. To date, an equivalent quantitative model for asphaltenes has not been proposed, in part because the magnitude of asphaltene particle cohesive force (Fa) is unknown. Eskin et al.25 and Maqbool et al.26 modelled asphaltene deposition for nano-sized particles but this information cannot readily be scaled up to an aggregating slurry in a flowline because nano-sized particles are smaller than the Kolmogorov length scale31 of about 1 micron,32,

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which dictates the force balance in

Equation (1). In this study, the experimental technique developed to determine Fa for hydrate particles was adapted and used, for the first time, to measure asphaltene particle cohesive forces in both liquid and gas continuous phases. When the data are interpreted through fundamental cohesive force models, the results enable direct scaling of the cohesive forces and aggregation mechanics relevant to a wide range of length scales.


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2 Methods And Materials 2.1 Asphaltene Precipitation To generate the asphaltene particles for study, 10 g of crude oil was diluted by 300 mL of nhexane (99% Sigma-Aldrich) based on the procedure described by Graham et al.9 While laboratory-precipitated asphaltenes may be extracted using several liquids, including hexane and heptane, only hexane-precipitated asphaltenes were used in the present study; the comparison between particles precipitated from different hydrocarbons remains an area for future work. To precipitate the asphaltene particles, the fluids were vigorously shaken to ensure complete dispersal of the oil in the solvent and allowed to equilibrate for 24 hours at ambient temperature to allow the solid asphaltene particles to precipitate. After 24 hours, the asphaltene solid suspension was filtered under vacuum through a 0.45 µm nylon membrane with the collected solids being allowed to dry in air for 24 hours at ambient temperature prior to use. Asphaltene particles were extracted from the filter paper and then visually inspected under an Olympus IX73 inverted light microscope to ensure the particles did not contain surface asperities larger than 10% of the particle diameter. Selected particles were attached to the tips of carbon fiber cantilevers with epoxy (bisphenol resin with tertiary amine hardener). Asphaltene particles were visually inspected with the microscope to ensure epoxy was absent from the asphaltene particle surface. In general, the precipitated asphaltene particle diameters were between 100 and 200 µm. Asphaltenes were precipitated using n-hexane from two Australian crude oils (A and B) for which the physical properties have been reported by Fridjonsson et al.34 Oils A and B had specific gravities of 0.90, 0.82 and asphaltene contents 0.42, 1.74 wt%, respectively. Oils A and


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B were respectively precipitated in triplicate and duplicate, resulting in multiple independent samples of asphaltene solid available for cohesive force measurements.

2.2 Micromechanical Force Measurements A third-generation MMF apparatus was used to directly measure the asphaltene interparticle cohesive force, using techniques adapted from previous research on hydrate cohesion and adhesion force studies.35,

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The apparatus consisted of an Olympus IX-73 inverted light

microscope equipped with digital recording equipment, where images were captured and analyzed via Olympus Stream Motion software (see Figure 2 for a picture of the MMF apparatus). An experimental cell (Figure 3) was placed atop the microscope stage, and was surrounded by temperature controlled aluminum jacket. This temperature control was achieved by circulating a 50-50 glycol-water mixture through the cooling jacket using a ThermoFisher cooling/heating bath (-30 to 80 °C). The experimental cell was filled with either air or liquid hexane (99% Sigma-Aldrich), and the cell temperature was monitored continuously by a thermocouple with an uncertainty of ± 0.5 °C during measurements. We note that the nearambient temperatures used in this study are not representative of the high-temperature wellbore conditions at which asphaltene solids are expected to precipitate in the field. Measurements at such conditions with the current apparatus would not be viable and/or likely have large uncertainties. Instead the temperature range was selected to allow direct comparison of the results to literature measurements of hydrate cohesive force. In future work we can investigate the effect of temperature under controlled conditions and use those results to infer the likely cohesive forces at well-bore conditions.


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Figure 2: Picture of the micromechanical force apparatus.

The microscope and cell assembly were placed atop of an active pneumatic Standa 1VIS10W vibration isolation table to minimize external interference with the particle contact. The cell contained two glass capillaries (1000 micron internal diameter) that were held by stainless steel arms. A carbon fiber filament (7 micron external diameter from Fibre Glast Developments Corporation) was secured within each capillary tube by epoxy adhesive. An asphaltene particle was affixed to the end of each carbon fiber filament (Figure 3-a). When assembled, the position of each asphaltene particle was controlled by a remotely-operated Eppendorf Patchman NP2 micromanipulator with sub-micron precision.


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Figure 3. (a) Top view schematic of experimental cell placed atop inverted light microscope where (b) a four-step pull-off procedure allows for direct calculation of the preload and cohesive force as the product of the spring constant (kspring) and preload or cohesive displacements (∆P, ∆D), respectively.

Cohesive force measurement between the particles were performed using the pull-off method described by Yang et al.28 (Figure 3-b): (i) the top particle was lowered onto the bottom particle, providing a pre-load displacement at a specified preload or contact force; (ii) the particles remained in contact for a specified time, which was typically 10 seconds; (iii) the top particle was then raised at constant velocity until the particles separated; and (iv) the displacement was captured visually by the Olympus Stream Motion recording software.

To calculate the pull-off force in this technique knowledge of the spring constant of the bottom cantilever was required. The spring constant of the carbon fiber cantilever was calculated from


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the measured geometric properties of the fiber (methodology outlined below) and reported elastic modulus of carbon fiber (250 ± 15 GPa37) using Equation (5):

(5)

where k is the spring constant, E is the elastic modulus of the material, d is the diameter of the fibre, and L is the length of the fiber. The elastic modulus, fiber diameter and fiber length had relative uncertainties of 6%, 7% and 0.1%, respectively. The elastic modulus of carbon fiber was reported from a study by Ilankeran et al.37 as 250 GPa. Typical lengths of the carbon fibers used were 2000-3000 µm, as measured to a 2µm accuracy under the Olympus microscope described above. To measure the fiber diameter, five representative carbon fibers were studied under a Scanning Electron Microscope. Measurements of the diameter of the fiber were taken along the fiber to assess the variance along and across different fibers. The average fiber diameter was determined to be 7 ± 0.5 µm. Propagating the uncertainties in these parameters leads to an estimate for the relative uncertainty of the calculated spring constant of 15%. The calculated spring constants of several carbon fiber cantilevers were checked experimentally as part of a benchmarking exercise as outlined in the following section.

The cohesive force of each pull-off trial was determined via Hooke’s law as the product of the final displacement and the spring constant of the bottom (horizontal) cantilever. The measured cohesive force was then divided by the harmonic mean radius of the particle pair (R*), calculated using Equation (6), to give the radius-normalized cohesive force that accounts for the scaling between force and particle size.


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(6)

In each experiment, at least 60 pull-off trials were performed to obtain an average cohesive force with an acceptable uncertainty, as described below. The average radius-normalized cohesive and preload force values measured here are reported together with a statistical uncertainty bound corresponding to one standard deviation (SD) of the measured distribution. When adapting the MMF apparatus for use with asphaltene particles, care was taken to ensure repeatability in the cantilever spring constant (Section 3.1) and the appropriate selection of preload force ranges (Section 3.2).

3 Results & Discussion 3.1 MMF Apparatus Benchmarking Each asphaltene experiment required a new particle pair and a new cantilever pair. To assess the accuracy of spring constants calculated using Equation (5), four carbon fibers were experimentally calibrated against a 24 mm long, tungsten wire with a diameter of 50.8 ± 1.3 µm and a known spring constant of 0.025 ± 0.003 N⋅m-1. The tungsten wire’s spring constant had been determined by direct force measurements using the procedure described by Taylor.38 The spring constants of the four carbon fibers were then measured relative to that of the tungsten wire using an indirect calibration method also described by Taylor.38 The results, shown in Figure 4, indicate that the calculated and measured spring constant were within the combined uncertainty


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of the theoretical spring constant. Implicit in the analysis of pull-off measurements is the assumption that the relationship between pull off distance and pull-off force is linear. In this preliminary investigation, the carbon fibers behaved linearly at deflections up to 15% of their length. The linear region may extend beyond this threshold, but was not required for the pull-off forces captured in this investigation. 0.015

Predicted Spring Constant (N/m)

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0.012

0.009

0.006

0.003

0

0

0.003

0.006

0.009

0.012

0.015

Measured Spring Constant (N/m)

Figure 4: Comparison of the measured and predicted spring constant for four carbon fibres using the relative calibration method from Taylor.38 The solid red line represents parity between measured and predicted spring constant with the red shaded region representing the uncertainty bounds of the calculated spring constant.

The Olympus Steam Motion software was used to convert pixels to physical length for cohesive force measurements. This ratio of distance to pixels was calibrated manually, using a stage micrometer with 10 µm divisions (Olympus). To ensure the apparatus was free of systematic offsets, the cohesive force between cyclopentane hydrate solids immersed in cyclopentane liquid was measured and compared with the results obtained by Aman et al.27 using a separate apparatus. A reference value of (4.3 ± 1.2) mN⋅m-1 was obtained by Aman et al.27 from 35 independent trials with cyclopentane hydrate particles ranging in size from 150 to 400


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µm at a temperature of 3.2°C, a pre-load force of 1.7mN⋅m-1, and a contact time of 10 seconds. Cyclopentane hydrate particles of similar diameter were prepared using the methodology described by Aman et al,27 where hydrates were formed by dissociating ice particles in the presence of bulk liquid cyclopentane. The particles converted to a cyclopentane hydrate shell within minutes, and were sufficiently solid for MMF experiments within 30 minutes of ice dissociation.27 The average cohesive force was measured across 10 experiments at the same temperature, and with identical preload force and contact time. The radius-normalized cohesive force for cyclopentane hydrates in cyclopentane liquids was measured to be (4.4 ± 1.6) mN⋅m-1, which is in excellent agreement with the value obtained with the entirely different MMF apparatus.

3.2 Asphaltene Cohesive Force The asphaltene interparticle cohesive force was measured as a function of preload force, to derive information about the asphaltene surface free energy39 and the extent of visible particle deformation under load.39 The effect of preload force was examined over the range 0.1 to 2 µN, and the asphaltene cohesive force was found to increase by a factor of two for both asphaltene A particles and asphaltene B particles (Figure 5). Each data point in Figure 5 represents at least two experiments with 60 pull-off trials each, where each experiment used a unique asphaltene particle pair. The three asphaltene A replicate measurements came from the three precipitation batches, and the two asphaltene B replicate measurements came from the two precipitation batches, as referenced in Section 2.1.


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Figure 5 Radius normalised cohesive (mN/m) force vs. preload force (µN) for asphaltene A and asphaltene B in air with a dashed line to guide the eye. Experimental conditions were a temperature and contact time of 25 °C and 10 s, respectively. The error bounds represent the standard deviation of the > 60 pull-off trials.

There are three key aspects to Figure 5: (i) asphaltene cohesive force increases linearly with preload force; (ii) with no preload force, the estimated cohesive force is non-zero; and (iii) asphaltenes A and B are indistinguishable within the resolution of this measurement. The linear dependence of cohesive force on preload force may be the consequence of semi-elastic particle deformation over the preload forces range studied. From a fundamental standpoint, this dependence may be analyzed from the two basic length-scales of interaction: intermolecular and interparticle.39 As the particle diameters exceed 10 microns, interparticle mechanics dominate the cohesive relationship through three primary mechanisms: solid-solid cohesion, capillary bridge cohesion, and sintering/growth between particles.39 Solid-solid cohesion is of primary interest in the present study, as there is neither a third phase available to form a capillary bridge or a growth at the contact point between the particles. The cohesive force between particles in a solid-solid cohesive mechanism will exhibit a dependence on the surface morphology and roughness. Johnson, Kendall and Roberts (JKR) and Hertz40 theory are theoretical treatments of the


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deformation of elastic spheres. Israelachvilli39 noted that both theories assume perfectly smooth surfaces, which may not be applicable to rough surfaces with asperities on the length scale of 1-2 nm.39 Surface roughness may affect the adhesive force between solids depending on the nature of the asperities and roughness. The asphaltene particles in this study exhibited asperities on the order of 10 µm, which were observed visually in the microscope; asperities below the optical resolution of the apparatus may have also been present. To account for the distributed surface roughness, average cohesive forces were obtained from repeat experiments with individual pairs of particles. In the solid-solid cohesive arrangement, two flat surfaces of species A contact each other in a continuous medium of species B. The free energy of cohesion (∆W) may therefore be equated with the energy penalty of generating interfacial area between species A and B, based on the interfacial tension (γAB). For a simple case of non-elastic spheres, this energy is defined by Equation (7): 39, 41 (7)

Solid particles obeying JKR or Hertz theory will deform directly with the magnitude of preload force, resulting in an increase in both interfacial contact area and cohesive force. By the physical basis for solid-solid cohesion described above, this condition translates to an increase in the global free energy required to separate the particles and creates asphaltene-air interfacial area. Israelachvili related this pull-off force to the interfacial energy between the particles and bulk medium-solid interfacial energy via Equation (8) below: .

(8)


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Here Fad is the force of adhesion, R is the harmonic mean radius of the particle pair and γSV is the solid-vapor interfacial tension. If the data from Figure 5 are extrapolated to a no-preload condition (without particle deformation), we hypothesize that the measured cohesive force at that condition is a function of the interfacial energy. From this value the interfacial asphaltene-air interfacial energy can be calculated to be 0.42 mJ/m2.

Figure 6 compares the calculated

asphaltene surface free energy to the free energy of carbon steel and cyclopentane hydrate in liquid cyclopentane.

Figure 6: Comparison of the surface free energies of common flow line solids. Carbon steel-air and cyclopentane hydrate-cyclopentane surface free energy is reported by Aspenes et al.42 and Aman et al.43 respectively.

The result in Figure 6 suggests that asphaltene surface free energy may be two orders of magnitude below that of carbon steel in air or cyclopentane hydrate in liquid cyclopentane. For a hypothetical flowline containing the interfaces identified in Figure 6, these data suggest that the formation of asphaltene interfaces may undergo a smaller energy penalty than the creation of a gas hydrate interface. Further studies are required to determine whether the MMF method at this resolution is capable of distinguishing between the interfacial energies of a larger matrix of crude oil asphaltenes. In particular, future studies will consider variable precipitation and purification


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methods to compare the cohesive forces and surface free energy of solids and surface active material that may co-precipitate alongside the asphaltenes.

3.3 Asphaltene Cohesion Variance with the Bulk Phase Hydrate investigations are typically performed in a bulk phase of liquid cyclopentane,27,

44

which has some characteristics of an oil phase and for which comparative data are available for hydrate particles. Figure 7 shows the average cohesive force obtained for the asphaltene particles in both a vapor and liquid phase compared to literature values for cyclopentane hydrate particles in similar phases and measured at comparable preload forces (1.7 mN/m).

Figure 7: Radius normalised cohesive force for Cyc5 hydrates in Cyc5 vapor (from Aman et. Al.44), Cyc5 hydrate in liquid Cyc5 (Aman et al.27), Asphaltene A in air at 25°C, and Asphaltene A in liquid Cyc5. Experimental conditions were 1.7 mN/m preload force and contact time of 10s. Uncertainty bars correspond to 1 SD.

When placed in liquid cyclopentane, the cohesive force of asphaltene A decreased by an order of magnitude relative to the cohesive force in air (Figure 7). The decrease in measured force reflects the fact that solid-vapor interfacial tensions (e.g. carbon steel-air at 66 ± 5 mN/m) 42 tend


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to be higher than solid-liquid interfacial tensions (e.g. ice-water at 31.7 ± 2.7 mN/m

45 39, 46

).

In

the liquid cyclopentane phase at the same experimental preload force and temperature, the asphaltene cohesive force was an order of magnitude lower than cyclopentane hydrate cohesive force. To contextualize this result, the measured asphaltene and hydrate cohesive forces were deployed in the slurry model proposed by Sinquin et al.30 (Equation (1)) to compare the maximum stable aggregate diameter for both solids under identical shear stress conditions. Representative asphaltene and hydrate particle sizes were estimated, respectively, to be 150 µm based on Ferworn et al.47 and 40 µm based on Sloan et al.29. The continuous phase was selected to be a live crude oil with a viscosity of 10 cP,30 and the volume fraction of each solid was set to 10%, where Sinquin et al.30 use a maximum random packing fraction of 57 vol%. A fractal dimension of 2.5 was estimated from Sinquin et al,

30

which requires direct experimental

validation for both asphaltene and hydrate aggregates. The resulting maximum stable aggregate diameter is shown in Figure 8 for variations in both shear rate and particle diameter.


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Figure 8: (top) Aggregate diameter as a function of shear rate for hydrate and asphaltene particles in crude oil; (bottom) aggregate diameter as a function of particle diameter at a constant shear rate of 100 1/s for hydrate and asphaltene particles in crude oil.

The result shown in Figure 8 demonstrates that hydrate particles aggregate more readily than asphaltene particles, even in the limit of small particle sizes. Varying the particle diameter as at a constant shear rate further illuminates the difference between hydrate and asphaltene aggregation potential, where the former is likely to dominate the flowline pressure drop signal. The substantially smaller cohesive force between macroscopic asphaltene particles suggests that they may not readily aggregate under typical flowline turbulence conditions, but may readily form large aggregates during non-flowing conditions.

3.4 Hydrate-Asphaltene Adhesive Force Work done by Gao48 identified that precipitated asphaltenes can interfere with the effectiveness of hydrate remediation technologies such as anti-agglomerants. Asphaltenes typically precipitate at temperatures and pressures above the hydrate equilibrium boundary, and dissolve slowly when compared to hydrate growth.6 For this reason, the co-existence of asphaltene and hydrate solids is kinetically feasible (see Figure 1), but there is no information


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available to inform the degree of synergistic cross-interaction between both types of solid. In production scenarios where both hydrate and asphaltene solids are simultaneously stable, there remains an open question as to whether the particles may synergistically aggregate.

The MMF apparatus is unique in its ability to study the interaction of two particles in isolation, and is well suited to inform this question. One cyclopentane hydrate particle and one asphaltene particle were contacted in a bulk phase of liquid cyclopentane as per the experimental methodology described above, with the same preload force (1.7 mN/m) and temperature (3.2 °C) used previously for baseline studies.

Figure 9: Radius normalised cohesive/adhesive force measurements for a Cyc5 hydrate particle pair in Cyc5, a hydrate–asphaltene particle pair in Cyc5, and an asphaltene A particle pair in CyC5. Experiments were performed at 1.7 mN/m preload force, 3.2°C, and a contact time of 10s. Uncertainty bars are 1 SD.

The resultant hydrate-asphaltene adhesive force reported in Figure 9 as the average of 40 pulloff trials. The result is counter-intuitive in that the hydrocarbon-based asphaltene particle did not reject the hydrate upon contact. Rather, the measured hydrate-asphaltene adhesive force was of the same order of magnitude reported for hydrate-hydrate cohesion. This observation may be the


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result of an intermediate wetting environment on the asphaltene surface, generated by the large polar molecules present within the solubility class. The capillary bridge model for hydrate cohesion reported by Aman et al.44 has three fundamental dependencies: (i) the bridge-bulk interfacial tension; (ii) the contact angle of the bridge on the particles; and (iii) the volume of the capillary bridge. For the hydrate-asphaltene cohesive contact measurements, we may assume that neither the interfacial tension nor the bridge volume changes appreciably. As such, the similarity between hydrate-hydrate cohesive and asphaltene-hydrate adhesive forces may be primarily ascribed to an intermediate wetting state on the hydrate and asphaltene particle surfaces in the presence of liquid cyclopentane. The comparison in Figure 9 raises an important question about the potential synergistic interaction between precipitated asphaltene and hydrate particles in subsea oil and gas pipelines; further characterization is required to determine whether the current standard assumption of non-interaction may result in an underestimation of combined hydrate plug formation risk and asphaltene deposition rate.


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4 Conclusions We report the first interparticle cohesive force measurements between precipitated asphaltene particles. Asphaltene was precipitated from two crude oils in n-hexane and filtered, generating solid particles with diameters ranging from 100 to 200 µm. The cohesive forces obtained between asphaltene particle pairs precipitated from two crude oils were indistinguishable; further evaluation of this technique with a large array of crude oils is required to determine the extent of variation in asphaltene cohesive force. In the vapor phase, asphaltene cohesive forces were approximately half the magnitude of those reported for cyclopentane hydrates. The asphaltene cohesive force was found to depend linearly on preload/contact force, increasing from 4 to 10 mN/m over a preload force range of 0.1 to 2 µN. When particles were placed in the same bulk phase of liquid cyclopentane, asphaltene cohesion was found to be approximately one order of magnitude lower than cyclopentane hydrate. These reduced interparticle cohesive forces for asphaltenes have significant implications for the maximum stable aggregate diameter that can be maintained in a flowing system. A preliminary investigation into the adhesive force between one hydrate and one asphaltene particle suggests that interparticle forces are on the same order of magnitude as hydrate cohesion. This result suggests that the current assumption of noninteracting hydrocarbon solids may require reevaluation to avoid underestimating the total blockage risk from hydrate and asphaltene formation.


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AUTHOR INFORMATION Corresponding Author * Corresponding author: [email protected], +61 8 6488 8561

Acknowledgments EFM acknowledges Chevron for the Gas Process Engineering Endowment at the University of Western Australia. ZMA acknowledges the University of Western Australia for a Research Development Award. The MMF apparatus was made available as part of the National Geosequestration Laboratory, funded by the Australian Government through the Education Investment Fund.

Abbreviations MMF, Micromechanical Force Apparatus; UWA, University of Western Australia; Cyc5, Cyclopentane; SD, Standard Deviation


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Micromechanical Cohesive Force Measurements between

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